1. Field of the Invention
The present invention relates to a nitride-type compound semiconductor laser device capable of emitting light from a blue range to a UV range, and further relates to a laser apparatus incorporating the same.
2. Description of the Related Art
Gallium nitride is a III-V group compound semiconductor material whose bandgap is as large as about 3.4 eV, and has been positively studied as a material to be used in a light emitting device capable of emitting light from a blue range to a UV range.
Referring to FIG. 8, a conventional gallium nitride-type compound semiconductor laser 40 will be described.
The semiconductor laser 40 of FIG. 8 includes a layered structure in which an n-type GaN buffer layer 22b, an n-type AlGaN cladding layer 23, an As-added GaN light emitting layer 54, a p-type AlGaN cladding layer 28 and a p-type GaN contact layer 59 are formed in this order on a 3C-SiC substrate 46. An n-type GaN current blocking layer 45 including a stripe-shaped opening is formed on the p-type AlGaN cladding layer 28. A p-electrode 10 and an n-electrode 11 are provided on the upper surface of the p-type GaN contact layer 59 and on the bottom surface of the 3C-SiC substrate 46, respectively. A current flowing from the p-electrode 10 to the n-electrode 11 is constricted by the n-type GaN current blocking layer 45. Such a gallium nitride-type compound semiconductor laser is disclosed in, for example, Japanese Laid-open Publication No. 7-249820.
However, it is not practical to form electrodes over the entire upper and bottom surfaces of the device, as in such a conventional gallium nitride-type compound semiconductor laser 40 illustrated in FIG. 8. Thus, in many cases, there are provided areas 52 along sides of the device which are not covered by the electrodes, as shown in FIG. 9.
FIG. 10 is a perspective view illustrating the conventional gallium nitride-type compound semiconductor laser 50 of FIG. 9 having the areas 52, as it is incorporated into a package. In FIG. 10, reference numeral 51 denotes a photodiode and reference numeral 53 denotes a back surface of the semiconductor laser device 50 which corresponds to the surface on which the electrode 10 is formed in FIG. 9. FIGS. 11A and 11B are schematic diagrams each illustrating the positional relationship between the conventional gallium nitride-type compound semiconductor laser 50 and the photodiode 51 incorporated in the same package illustrated in FIG. 10. Specifically, in FIGS. 11A and 11B, the photodiode 51 is viewed from directions indicated by arrows A and B in FIG. 10, respectively.
In many cases, the photodiode 51 is provided on the side of the back surface 53 of the semiconductor laser 50, as illustrated in FIG. 10, for detecting the output from the back of the semiconductor laser 50 and for controlling the semiconductor laser 50 based on the detected output. A layered structure constituting such a conventional gallium nitride-type compound semiconductor laser is transparent to the emission wavelength. Therefore, as illustrated in FIGS. 11A and 11B, the light generated in the active layer may partially leak out of the device through the areas 52 which are not covered by the electrode 10 and be incident upon the photodiode 51 as noise, thereby causing an erroneous control.
Moreover, since such a package typically includes a cap welded thereto for airtight sealing, the leaked light impinges on the inner wall of the cap and causes multiple reflection. Thus, even when the photodiode 51 is provided at a position other than on the side of the back surface 53 of the semiconductor laser 50 as illustrated in FIG. 10, such leaked light may still be incident upon the photodiode 51 as noise, thereby causing an erroneous control.